Mechanism Of Fracture Propagation Induced By Hydraulic Fracturing In Naturally Fractured Shale:Implications For Applied Research

As an effective means of fracturing the rock mass,hydraulic fracturing holds a promising prospect in extracting unconventional oil and gas resources such as shale gas.In addition,it can also be applied in many other fields.However,fracture propagation induced by hydraulic fracturing in naturally fractured shale is rather complex due to anisotropy.Meanwhile,the location of fluid injection in hydraulic fracturing is also critical for the formation of hydraulic fracture network.The inappropriate selection of the fluid injection location can result in fault activation thereby causing an evident microseismic effect.Therefore,it is of great realistic significance to investigate the mechanism of fracture propagation induced by hydraulic fracturing in naturally fractured shale.Using field investigation,laboratory experiment,numerical modeling,and theoretical analysis,this paper carried out a systematic study of the propagation of hydraulic fractures in naturally fractured shale.This study focuses mainly on: a)the sensitivity of the hydraulic fractures to the naturally occurring discontinuities(NOD);b)the mechanism of the interaction of the hydraulic fractures with the NOD;c)the evolution characteristics of the hydraulic fracture network;d)the formation of the microseismic effect and the optimization design of the fluid injection location in hydraulic fracturing.The novel results of this study are summarized as follows:(1)This study revealed the sensitivity of the fractures to the parameters of the NOD during hydraulic fracturing of anisotropic shale.According to the changes of the mechanical properties of the anisotropic shale,this study constructed a particle-contact discrete fracture network to characterize the mineralogical composition of the shale.The effect of the parameters of the NOD(such as strength,ratio of cohesion to tensile strength,normal stiffness,shear stiffness,and occurring angle)on the tensile strength of the shale,the split modulus,the number and percentage of the fractures of different types,and the fracture pattern,etc.,were separately investigated.Moreover,this study proposed a method to select the parameters for the naturally fractured shale model.(2)This study made clear the interaction mechanism of the NOD with the hydraulic fractures.This study put forward the criterion for hydraulic fracture initiation in naturally fractured shale,explored the correlation of the initiation fracture with the initiation location,and established the mechanical model characterizing the interaction between the hydraulic fracture and the NOD.In addition,the mechanical model was validated by the hydraulic fracturing test on the anisotropic shale.Based on this model,this study investigated the interaction mechanism between the hydraulic fractures and the NOD and revealed the effect of the intrusion angle,the confining pressure,and the friction coefficient on the interaction mechanism.With the comprehensive investigation of the features such as breakdown pressure,circumferential displacement,and fracturing efficiency,this study shed light on the main reason for the differences of the mechanical effect caused by shale anisotropy.(3)This study established the fluid-solid coupling model for discrete fracture network and revealed the effect of the two structural parameters of the NOD on the propagation of the hydraulic fracture network.Based on the fluid-solid coupling algorithm for naturally fractured rock mass,this study developed a method to track and locate different types of hydraulic fractures.The effects of the structural parameters of the parallel NOD(such as azimuth angle,intermittence,spacing,step angle,and aperture)on the distribution and propagation of the hydraulic fracture network were systematically investigated.Based on the evolution pattern of the hydraulic fracture network,this study investigated the main factors affecting hydraulic fracturing efficiency and controlling hydraulic fracture propagation.Further,the effect of the non-parallel NOD in different conditions on volume fracturing,the gradient of breakdown pressure,and distribution and evolution of hydraulic fractures were also revealed.As a result,this study made clear the weakening mechanism of the density of the non-parallel naturally occurring discontinuity.(4)This study developed a method for quantitative evaluation of fault activation and microseismic events during hydraulic fracturing process and put forward an optimization method for the determination of the optimal fluid injection location in naturally fractured shale containing faults.This study introduced the moment tensor solution to calculate the magnitude of the microseismic events,established a microseismic effect model to evaluate the propagation of the hydraulic fracture network,and proposed a criterion to determine the occurring time and location of the same microseismic event.The proposed model was proved to be accurate by experimental data and theoretical calculation.Furthermore,this study performed quantitative analysis on the effect of the fluid injection location on fault slip distance,activation range,the magnitude of injection pressure,distribution of microseismic event magnitude,cumulative event frequency,and the number of fractures.The selection of the fluid injection location was also analyzed.The results of this study bear great significance in perfecting the theory of fracture propagation in reservoirs in the nonconventional oil and gas field and in guiding the mass application of hydraulic fracturing technique.This study can also provide some insight into the thorny issues plaguing the coal industry,such as hard roof breaking and weakening,seam permeability enhancement,gas extraction and management.Moreover,the results of the study can also be applied in harnessing deep geothermal energy.